This invention relates to a front fork for a bicycle and more particularly to a front fork consisting of members jointed in an inventive manner.
Front forks for bicycles using tubes and plates have been generally used. One example is shown in FIG. 1, wherein a front fork consists of a stem tube 8, a lug 9 and fork tubes 10. The lug 9 is made of a steel by press-forming or lost wax process. The stem tube 8 and the fork tubes 10 are fitted in the lug 9. The lug 9 is heated at 900.degree.-1000.degree. C. as a whole to melt brazing alloy 11. The brazing alloy 11 is then solidified to joint these members.
FIG. 2 illustrates another example of a front fork which has been made by the assignee of this application with the aid of die casting. Fork tubes 14 are integrally made in a unitary body as shown in phantom lines and are jointed to a stem tube 12 by arc-welding. The jointed fork tubes and stem tube are then set in metal dies and a lug 13 is formed by die casting.
In a further hitherto used front fork (not shown), fork tubes are formed by bulging or press-forming and directly jointed to a stem tube by arc welding such as tig welding without using a lug.
The lastly mentioned front fork jointed by welding has been applied to particular uses owing to the shape as a whole and appearance of the jointed portion. On the other hand, the front forks shown in FIGS. 1 and 2 have been widely used because they are sufficiently superior in strength and appearance with particular consideration.
Moreover, U.S. Pat. No. 4,479,662 discloses a bicycle frame comprising tubes having tapered portions extending therefrom and adapted to be fitted in inner tapered portions of mating tubes and adhesive substance for jointing the fitted tapered portions.
The above front forks hitherto used have the following disadvantages.
With the front fork disclosed in FIG. 1, the lug 9 jointed by brazing is made of iron (steel) by pressing or lost wax process and the fork tubes 10 cannot be made of a light metal such as aluminum. Accordingly, it is impossible to make light the front fork. Moreover, the shape of the lug 9 made by pressing is limited to a uniform shape resulting from limitations of the producing method. Suitable clearances are required at the fitted portions of the tubes for the brazing alloy, so that the respective members must be worked with high accuracy before brazing. Furthermore, the respective members themselves are heated to the brazing temperature so that portion of the members which need not be heated are exposed to the high temperature for a long period of time to lower their strength. As a flux is used for brazing, after-treatment such as pickling, finishing or the like is required so that the time for the operation is elongated and use of a combination of different materials is difficult.
With the front fork using die casting disclosed in FIG. 2, the fork tubes 14 are a unitary body and the tubes 12 and 14 are insert-cast in the lug 13 so that the lug 13 becomes bulky and awkward. Therefore, an inner lug system cannot be employed and a smart and light appearance cannot be obtained.
Moreover, as the fork tubes 14 are insert-cast by the lug, wall thickness of the fork tubes 14 is required for resisting the casting pressure so that it is difficult to make light the frame and to employ different materials for the fork tubes.
In the front fork disclosed the U.S. Pat. No. 4,479,662, there are the tapered portions of the lug and fork tubes at their fitted portions. Therefore, cutting and expanding the tubes at the fitted portions are required to increase processing steps and to increase production cost resulting from the requirement of working accuracy.